tastefulness and serviceability the most precious works of art in gold and silver, which are of necessity the exclusive property of the rich. The recent great improvements in dynamo-electric machines, which have been so far perfected as to have almost completely replaced the troublesome and expensive chemical battery for the electro-deposition of metals, have given an immense impulse to the art, and bid fair to work an important revolution in the metallurgical arts.

PART I.

THIN METALLIC DEPOSITS.

CHAPTER I.

OBJECT OF THE HYDROPLASTIC ART.

THE art of depositing metals by the wet way comprises two orders of phenomena, which, though apparently identical in their results, are nevertheless essentially different in respect to the causes of their production. The one, as, for example, gilding by simple immersion in a liquid or bath prepared for the purpose, is directly the result of the action of chemical affinities; the other, on the contrary, as, for instance, electro-gilding, cannot take place without the simultaneous concurrence of chemical reactions, and of that form of energy called dynamic electricity, which, though only recently discovered, has become of great practical importance.*

*Physicists recognize two modifications of electricity. They call static electricity that which, when accumulated on an insulated conductor, as, for example, in the prime conductor of an electrical machine, or in the Leyden jar, manifests its presence, in the form of sparks, on the approach of another conducting body.

They call, on the other hand, dynamic electricity, or current electricity, that modification which manifests its effects with or without the disengagement of light, as, for example, when contact is established between the two electrodes of a galvanic battery, or when a conductor of any description is interposed between the extremities of these reophores. This subject will be more fully discussed in the chapter on Batteries.

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The frequent use in the same workshop of metalloplastic methods, both with and without the aid of electricity, renders it necessary that we should not separate these two orders of facts; in support of which opinion we have the additional reasons that the manipulations and chemical preparations are very nearly identical in both cases, and the results obtained are sensibly the same.

In the operations of deposition by simple immersion, where the dissolved metals are reduced to their metallic state by the sole agency of chemical affinities, the results obtained are, in most cases, very similar, the metallic deposits thus obtained being generally characterized by their extreme thinness.

On the other hand, we may by electro-deposition attain two different objects!

It may be desired, for instance, to coat a common metal with a layer of another metal which is more precious, more resisting, less oxidizable, or which has a more pleasing appearance; but this layer, generally, borrows its strength from the underlying metal, as is the case in gilding or silvering copper, or in coating zinc with copper,

etc.

Or, we may design to reproduce a given article with scrupulous exactness, by means of a metallic deposit, which, after its separation from the mould, shall have sufficient firmness to constitute a fac-simile of the original; of this nature are the galvanoplastic reproductions of basso-relievos, medals, etc. Or, we may wish to impart a metallic hardness to a material without firmness, and retain at the same time as nearly as possible its original shapes and dimensions, as when we deposit metallic coatings upon small statues, flowers, fruits, insects, etc.

We shall examine first the thin electro-deposits, and

principally those of gold and silver, as their applications present great variety, and are of general interest.

The following definitions may be found useful to students of this subject:

Electro-chemistry is that branch of the science of chemistry which treats of the agency of electricity in producing chemical changes. The branch of electro-chemistry with which we are here specially interested is,

Electro-metallury, which deals with the deposition of metals from their compounds held in solution, by the agency of galvanic (or current) 'electricity. According to the object sought to be attained, the following subdivisions are recognized:

Electro-metallurgy proper, which relates to the extraction of metals, on the commercial scale, from solutions of their ores or other compounds, by means of the galvanic current, the resulting product being placed in merchantable form (ingots, bars, sheets, wire, etc. etc.).

Electrotyping, by which term is understood the reproduction and duplication of useful and artistic objects (medallions, coins, statues, and other works of art), and the reproduction and duplication of printing surfaces (engravings, letter-press) in copper, by means of the galvanic current. The resulting electro-deposited metal is usually removed from the surface on which it has been deposited.

Electro-plating, which is confined to the coating of any metallic or other conducting surface, by means of the galvanic current, with an adhering deposit of another metal, either for the purpose of enhancing the beauty and value of the object thus treated, or to protect the underlying surfaces from the action of corrosive agents, or for both of these purposes.

CHAPTER II.

CLEANSING.

CLEANSING COPPER AND ITS ALLOYS-CLEANSING BY FIRE OR BY ALKALIES-DIPPING-DIPPING IN OLD AQUA FORTISDIPPING IN AQUA FORTIS AND SOOT-AQUA FORTIS FOR BRIGHT DIPPING-BRIGHT DIPPING IN MIXED ACIDS-WHITENING BATH-MIXED ACIDS FOR A DEAD LUSTRE-DIPPING IN NITRATE OF MERCURY.

BEFORE undertaking to lay a thin electro-deposit of one metal upon another, it is absolutely necessary that the latter should be chemically clean, as the presence of the slightest impurity is sufficient to prevent the adherence of the two metals.

The operation or series of operations by which this result is reached is called cleansing.

The mode of cleansing varies with the metals operated upon; the cleansing of copper and its alloys presents the greatest difficulties, and requires to be done with the greatest care and thoroughness. The cleansing of other metals, such as silver, iron, lead, tin, etc., is more a mechanical than a chemical operation.

We shall examine successively the preparation which metals and their alloys should receive before they are ready for a metallic deposit, following substantially the mode of procedure recommended by Roseleur.